6.3.4 Analyzing sender and receiver reports

6.3.4 Analyzing sender and receiver reports

It is expected that reception quality feedback will be useful not
only for the sender but also for other receivers and third-party
monitors. The sender may modify its transmissions based on the
feedback; receivers can determine whether problems are local,
regional or global; network managers may use profile-independent
monitors that receive only the RTCP packets and not the corresponding
RTP data packets to evaluate the performance of their networks for
multicast distribution.

Cumulative counts are used in both the sender information and
receiver report blocks so that differences may be calculated between
any two reports to make measurements over both short and long time
periods, and to provide resilience against the loss of a report. The
difference between the last two reports received can be used to
estimate the recent quality of the distribution. The NTP timestamp is
included so that rates may be calculated from these differences over
the interval between two reports. Since that timestamp is independent
of the clock rate for the data encoding, it is possible to implement
encoding- and profile-independent quality monitors.

An example calculation is the packet loss rate over the interval
between two reception reports. The difference in the cumulative
number of packets lost gives the number lost during that interval.
The difference in the extended last sequence numbers received gives
the number of packets expected during the interval. The ratio of
these two is the packet loss fraction over the interval. This ratio
should equal the fraction lost field if the two reports are
consecutive, but otherwise not. The loss rate per second can be
obtained by dividing the loss fraction by the difference in NTP
timestamps, expressed in seconds. The number of packets received is
the number of packets expected minus the number lost. The number of
packets expected may also be used to judge the statistical validity
of any loss estimates. For example, 1 out of 5 packets lost has a
lower significance than 200 out of 1000.

From the sender information, a third-party monitor can calculate the
average payload data rate and the average packet rate over an
interval without receiving the data. Taking the ratio of the two
gives the average payload size. If it can be assumed that packet loss
is independent of packet size, then the number of packets received by
a particular receiver times the average payload size (or the
corresponding packet size) gives the apparent throughput available to
that receiver.

In addition to the cumulative counts which allow long-term packet
loss measurements using differences between reports, the fraction
lost field provides a short-term measurement from a single report.
This becomes more important as the size of a session scales up enough
that reception state information might not be kept for all receivers
or the interval between reports becomes long enough that only one
report might have been received from a particular receiver.

The interarrival jitter field provides a second short-term measure of
network congestion. Packet loss tracks persistent congestion while
the jitter measure tracks transient congestion. The jitter measure
may indicate congestion before it leads to packet loss. Since the
interarrival jitter field is only a snapshot of the jitter at the
time of a report, it may be necessary to analyze a number of reports
from one receiver over time or from multiple receivers, e.g., within
a single network.